[StreetChief]

If a system takes action, or affects something, those effects, or lack of, are notable. For an electronic system it's simple - it is powered on.

[edit] You can also consider the transient and steady states:

[1] steady state: https://en.wikipedia.org/wiki/Steady_state

[2] transient https://en.wikipedia.org/wiki/Transient_state

[edit]

My comment about the start doesn't actually address your concern about the end of the transient state. I think there are options. Any observable result can be considered the output of the system.

[edit]

Yoda would say "do or do not, there is no try." I prefer "there is only trying."

[edit]

i need coffee >.<

[j7ake]

Your answer does not address the time component.

Take an example: an electronic system is designed to give a solution to the traveling salesman problem on a list of a billion of destinations.

You turn the system “on” so it can start doing the calculation.

How long should you have to wait to get your desired answer? It may take longer than the lifetime of the universe.

Until the system spits out an answer, the electronic system is no different than a system that just sits there and does nothing.

[StreetChief]

As you describe it, the system is worthless then and does nothing. How long to wait is up to you.

https://en.wikipedia.org/wiki/Halting_problem

[Edit]

Any observations can be an event, and a stopping point.

[Edit] I could put a resistor on the table and call it a system, and wait for it eternally, but I'm not sure that's useful. You need a desired outcome, and some limits to make rational decisions. Actually, mathematically, we can find certain limits.

[Edit]

You also can't prove a negative, as some future state may eventually disprove it. So you will have to assert something like "I expect such results at such time."